Many ophthalmic surgical procedures require illuminating a portion of a patient's eye so that a surgeon can observe a surgical site. Various different types of instruments are known and available for use by an ophthalmic surgeon to illuminate the interior of the eye. For example, the handheld (probe) portion of a typical ophthalmic illuminator comprises a handle having a projecting tip and a length of optical fiber that enters a proximal end of the handle and passes through the handle and the tip to a distal end of the tip, from which light traveling along the optical fiber can project. The proximal end of the optical fiber can be optically coupled to a light source, such as in a high brightness illuminator, to receive the light that is transmitted through the fiber. These types of handheld illuminators are typically used by inserting the probe tip through a small incision in the eye. In this way, light from the illuminator light source is carried along the optical fiber, through the handpiece and emitted from the distal end of the probe (fiber) to illuminate the surgical site for the surgeon. Ophthalmic illumination systems that use a length of optical fiber to carry and direct light from a light source to a surgical site are well known in the art.
Such ophthalmic illumination systems typically comprise a handheld portion including a probe, to deliver illumination from a light source housed in an enclosure. The enclosure typically houses the light source and associated optics that guide light from the light source to the optical fiber of a probe, a power supply, electronics with signal processing, and associated connectors, displays and other interfaces, as known to those having skill in the art. While some ophthalmic illumination systems use other types of lamps as a light source, a preferred light source is a xenon lamp.
An ophthalmic illumination system xenon lamp typically has a relatively small arc (e.g., about 0.18 mm width for an Osram 75 W xenon bulb at zero hours operating time). Optics within the illumination system are used to focus an image of the arc onto the optical fiber of the probe and the xenon bulb must be precisely aligned to ensure that an optimum amount of light is coupled into the optical fiber, and hence an optimum luminous flux emerges from the fiber. The optical fiber core diameter is selected to be large enough that the arc image will fit within the fiber core area. However, as the xenon bulb ages, the bulb cathode degrades and moves away from the bulb anode. As the cathode degrades, the arc grows in size, decreases in peak luminance and the arc center moves away from the anode.
The xenon bulb is positioned so that the arc image will fall on the optical fiber core entrance surface. In prior art illumination systems, the xenon bulb is positioned such that maximum fiber throughput is achieved at zero hours of operation (i.e., beginning of life of the xenon bulb). However, the arc can move (due to cathode degradation) in excess of about 250 microns during the first 200 hours of operation in a typical illumination system. Therefore, if the xenon bulb is aligned for maximum fiber throughput at zero hours, the arc movement (which can result in much of the arc image moving outside of the fiber core area) combined with the decrease in arc peak luminance will result in an appreciable drop in fiber throughput, and hence in an appreciable drop in illumination at the surgical site.
One way of solving this problem in prior art ophthalmic illumination systems is to increase the diameter of the proximal end of the optical fiber core. However, increasing the diameter of the optical fiber has several disadvantages. One disadvantage is that the increased fiber diameter results in a stiffer optical fiber, which is not as easy to manipulate in an operating environment. Further, a larger diameter fiber is more expensive because more fiber material is used per unit length of optical fiber. Even further, a larger diameter fiber may be greater than that allowed by the probe requirements. For example, a 20 gauge ophthalmic illuminator probe (0.355 inch cannula outer diameter) can accommodate a maximum diameter of the fiber core and cladding of 0.0295 inches. Further still, undesired dissipation of the light from the light source can result from allowing a tightly focused arc image to expand into a larger diameter beam as defined by the larger diameter fiber. Once this light concentration is lost, it cannot be recaptured. If the optical fiber tapers to a smaller diameter downstream from its proximal end, the ability of the light to efficiently transmit through the tapered fiber will depend on the concentration of the light prior to the start of the fiber taper. If the arc image is allowed to spread spatially and dissipate its light concentration, the light will transmit less efficiently into the tapered fiber portion.
Therefore, a need exists for a method and system for enhancing the useful lifetime of an ophthalmic illumination system that can reduce or eliminate the problems of prior art ophthalmic illumination systems discussed above.